Some mistakes that i have in mind are( iwill not mention continuity problems that appear to the movie only based what ady did):

Mostly the new backgrounds have no moving grain. It yells static image.
When luke enters his speeder to search the uncles when the fading starts, the sky mask is messed up.
When we see Luke with his saber in MF after the destruction of the death star the colors of the panels are not match with the next same shot. (appears to the SE too)
Most of the masks are very blurry sometimes causing some weird effects. Several examples are sky replacements and rotoscoping.
Even in the comparison images by adywan there is a shot with a missing rotoed part when a tie fighter explodes outside the MF gunner cockpit. You can see the gray explosion via the yellow explosion.
Maybe artistic liberation but R2's eye is cyan and not blue.
When the stabilizers are broke lose again, biggs yells to luke, when ben says use the force,let go,trust me R2 is again Black and white
Vader green button changes during the movie to blue and faded green
During the dune sea scenes it appears a weird green tint over the sand. Green remainings also can be located to deathstar trench sequences
When the MF enters the hangar the surrounding details are not match with the rest of the shots

Thats for now :)

-Angel